OBJECTIVE: To characterize health insurance gap patterns related to age-19 Medicaid and age-26 commercial age-eligibility cutoffs. STUDY SETTING AND DESIGN: This descriptive analysis spans 2014-2018, after Affordable Care Act implementation, but before COVID-19 emergency provisions. We defined insurance gaps as ≥3 consecutive months without observed enrollment, preceded and followed by ≥1 month of enrollment and stratified results by insurance source and clinical severity (e.g., with chronic, complex, or disabling conditions or not). DATA SOURCES AND ANALYTIC SAMPLE: The Colorado all-payer claims database provided data for enrollees aged 10-29, 52% (649,346) of whom were initially Medicaid insured, whereas 47% (576,596) were commercially insured. PRINCIPAL FINDINGS: The percent of enrollees with insurance gaps peaks within six months of turning age-19 and age-26-at 8.9% Medicaid and 8.7% commercial, respectively. The percentage point difference between ages 27-28 and 11-18 was 3.3 percentage points higher for prior Medicaid recipients (p <
0.001) and 2.2 percentage points greater for prior commercial enrollees (p <
0.001). Relative to the other clinical severity groups, young adults with disabling health conditions who were initially Medicaid insured had the lowest peak gap rate, 5.7%, compared with 10.5% among the previously commercially insured
this latter finding was sensitive to gap specification. CONCLUSIONS: Young adults would likely benefit from greater attention to age-19 and age-26 health insurance "unwinding."